1. Field of the Invention
This invention relates to an ultrasonic flowmeter with a measuring tube through which flows a medium and which, viewed in its cross section, has a bipartite split circumference forming two halves, and with two pairs of ultrasonic converters, each with an associated ultrasound reflector, which ultrasonic converters of each ultrasonic converter pair are mounted on a common circumferential half in a mutually offset position as viewed in the longitudinal direction of the measuring tube, while the ultrasound reflector for each pair of ultrasonic converters is positioned on the other, opposite circumferential half of the measuring tube between the two ultrasonic converters, in such fashion that an ultrasound signal emitted by one ultrasonic converter of an ultrasonic converter pair travels along a V-shaped signal path via the ultrasound reflector of the associated ultrasonic converter pair to the other ultrasonic converter of that ultrasonic converter pair.
The measuring accuracy of ultrasonic flowmeters generally tends to deteriorate when the medium passing through the measuring tube flows unevenly, thus deviating from a fully developed laminar or turbulent flow pattern. Such irregularities may be due to changing pipe diameters or bends along the flow path of the medium. These irregularities are generally differentiated among three categories, to wit: axial disturbances, tangential disturbances such as vortices, and radial disturbances. Tangential and radial disturbances do not contribute to a change in the actual volume flow rate. They do, however, affect ultrasonic measurements, as will be explained below.
An ultrasonic flowmeter typically encompasses at least two ultrasonic converters, together constituting an ultrasonic converter pair and mounted in a mutually offset position in the direction of the flow. One of the ultrasonic converters emits an ultrasound signal that passes through the flowing medium and is received by the other ultrasonic converter. The system design usually provides for the other ultrasonic converter to be able on its part to transmit an ultrasound signal that is received by the first ultrasonic converter. In this fashion, ultrasound signals alternately pass through the flowing medium in or against the direction of flow. The entrainment effect of the flowing medium results in different runtimes in and, respectively, against, the flow direction. Where the length and the angle of the acoustic path relative to the flow direction are known, the runtimes in and against the flow direction permit a determination of the flow rate of the medium.
If a pipeline includes, for instance, an upward or downward bend followed by a bend to the right or left, the flow pattern can be expected to be subject to axial as well as tangential irregularities. Moreover, the tangential disturbance is likely to add another speed component along the acoustic measuring path, thus falsifying the runtimes on the basis of which the overall flow rate is determined.
2. The Prior Art
To date, the approach for a solution to this problem has been to employ a minimum of two mutually intersecting measuring paths in one common plane. If the angles of the two measuring paths in the direction of flow are identical, the undesirable tangential and radial speed components can be eliminated by averaging the flow rates determined along those two paths.
One drawback lies in the fact that at least twice as many ultrasonic converters are needed. Accommodating additional ultrasonic converters in the measuring tube usually requires additional converter pockets in the wall of the measuring tube which, in turn, adds further flow disturbances.
Another approach to solving the above-described problem has been to use V-shaped signal paths. The method employed provides for the two ultrasonic converters of an ultrasonic converter pair to be mounted on one common side of the measuring tube, with an ultrasound reflector positioned on the opposite side of the measuring tube. Depending on the curvature of the measuring tube in the respective plane, such an ultrasound reflector may, in fact, be constituted of the inner wall of the measuring tube itself, or a separate ultrasound reflector may be installed for instance in the form of a flat plate. This is part of the prior art and also applies to the invention described further below.
In this case, an ultrasonic signal emitted by an ultrasound converter travels along a V-shaped signal path via the ultrasound reflector to the other ultrasound converter of the ultrasonic converter pair. Similarly, an ultrasonic signal can be transmitted in the opposite direction. As stated above, this makes for a configuration that permits the elimination of the tangential and radial components through an averaging process. As one major advantage of this solution, no additional ultrasound converters are needed.
US 200410011141 describes an ultrasonic flowmeter of the type described above. That device uses multiple V-shaped signal paths extending along mutually parallel planes. The ultrasound converter pairs and, respectively, the ultrasound reflectors are mounted on different sides of the measuring tube in the ultrasonic flowmeter. One advantage of that design is that at least the uppermost and the bottom-most V-shaped signal paths can be positioned at only a small maximum distance from the inner wall of the measuring tube. It has, in fact, been found that a minimal distance from the inner wall of the measuring tube is conducive to improved measuring accuracy since this type of signal path permits highly precise detection, and thus elimination, especially of axial irregularities. Moreover, jointly positioning all of the ultrasound converters on one single side of the measuring tube facilitates maintenance, especially when the measuring tube is accessible only with great difficulty and perhaps only from one side.
Using multiple V-shaped signal paths in mutually parallel planes, as described above, has proved to be substantially more effective in eliminating radial and tangential flow disturbances than is attainable with the conventional signal-path configurations referred to above. However, tests have revealed that there is still an error rate of about 0.15%.